7.1 DTS-MA Question

The rears get downmixed into the surrounds in a system configured as 5.1. You don't lose anything.

Actually, it might be a little more correct to say that the 5.1 'downmix' contains as much of the 7.1 version as the mixer/author decided. In this [slightly dated] DTS-HDMA downmix panel (exemplar), you can see (in the upper half) that in downmixing from 7.1_Standard to 5.1_Standard, the content in the L+R Rear Surrounds channels is just added to the content of the L+R (Side) Surrounds channels, and L|C|R|LFE content is unchanged, in this case. This seems [to me] like it might be a reasonable 'default' for most mixdowns.

However, the lower half of the panel shows that in the 5.1_Standard to 2.0 mixdown, the LFE signal is completely discarded.

TrueHD and dts-MA each use different methods of producing 5.1 outputs from 7.1 encodes. I suspect neither is technically a downmix. But, neither of them discards the rear channels. The 5.1 output includes the material in the 7.1 output, properly mixed for a 5.1 presentation. Since there are almost no 7.1 theatrical mixes, most of these soundtracks actually begin as 5.1 and get remixed for 7.1.

That's interesting about Toy Story 3. TS3 is the first movie mixed using Dolby Surround 7.1. Perhaps it was easier for the studio to simply encode the 7.1 theatrical mix as one track and the 5.1 version that played in most theaters as another, rather than integrating both mixes into a single dts-MA package. But, that's just speculation on my part.